Handling Pathological Input Data Securely When Using Sed

Mitigating Security Risks when Using sed

The stream editor sed is a powerful tool for text manipulation and transformation. However, if used carelessly, sed can introduce security vulnerabilities into applications that allow arbitrary user input. Malicious users may attempt to inject sed control sequences or exploit the evaluation of unsafe functions in an attempt to compromise the hosting system.

When developing sed-based text processing scripts and utilities, it is therefore essential to employ safe coding practices that restrict what sed can do based on untrusted input. This article outlines techniques to handle pathological user input securely when using sed in scripts and programs.

Sanitizing User Input to Prevent Code Injection

Sed allows users to specify editing commands to match and transform text via regular expressions. However, if unconstrained input is passed directly to sed, attackers could injecttheir own unauthorized sed control sequences to manipulate or exfiltrate data.

Input validation and sanitization should therefore be applied to remove potentially dangerous characters and syntax before interpolated into sed scripts. Techniques include:

• Filtering input to whitelist acceptable alphabet characters
• Escaping or removing sed metacharacters such as /, $, :, ! from user strings
• Parsing with regular expressions to reject input containing sed commands
• Employing allow lists of permitted sed functions rather than a deny list

Likewise, when generating sed expressions dynamically from external input, care should be taken to avoid concatenating strings in ways that could allow injection of unintended commands. Where possible, interpolate data separately from control syntax.

Quoting Metacharacters to Prevent Unintended Interpretation

Even where input has been sanitized, passing user strings to certain sed operations can lead to unexpected behavior if metacharacters are interpolated without quoting. For example:

sed "s/$userInput//"

Here, a / character could truncate the substitution command. Similarly, inserting unquoted strings into regular expressions may introduce . ^ $ * + etc, altering the match logic in unintended ways.

To avoid this, enclose external strings in quotes when referenced in sed scripts to protect metacharacters from re-interpretation:

sed "s/'$userInput'//"

Likewise, when inserting dynamic content into regular expressions, ensure it is quoted properly. This prevents markup within the string from being processed incorrectly:

sed -e "/'$userString'/d" 

Avoiding Evaluation of Unsafe Functions

Sed allows calling external programs and shell commands for additional text manipulation via the r, w and ! operators. However if malicious input contains embedded code, this could lead to arbitrary command execution.

If using such operators, input should be strictly sanitized beforehand to prevent injection. An allow list can specify only safe programs deemed necessary. Often however, external functions are better avoided entirely in favor of native sed functionality where possible.

Likewise, if passing dynamic sed scripts via the -e option, validate the code string to restrict allowable syntax and functions as a defense-in-depth measure.

Sandboxing sed Executions

As a privilege separation measure, consider invoking the sed interpreter inside a sandbox environment with restricted permissions. This confines the process in case flaws allow attackers to break out of the sed control domain.

Operating system enhancements like SELinux and AppArmor allow enforcing Filesystem access policies, executable mapping, network usage and other constraints on a per-process basis. Configuring a locked-down profile thus limits damage potential.

Likewise containerization platforms like Docker can isolate sed into a disposable container with only necessary data volumes mounted. This serves as a lightweight sandbox without excess libraries/devices available if compromised.

Employing AppArmor Profiles

On Linux systems, AppArmor mandatory access controls allow administrators to specify custom security policies enforced at the kernel level. By default, sed runs with unconfined access – an AppArmor profile can instead restrict things like:

• File reads/writes to approved directories containing input/output data
• Network access restrictions
• Access to executable mappings limited to the sed binary only
• Linux capabilities like loading kernel modules blocked

Developing a narrowly-scoped policy thus limits damage if the sed process gets hijacked. The aa-genprof tool can automatically generate an initial profile to refine further:

aa-genprof sed

Running as Unprivileged User for Least Privilege

Sed scripts are commonly used for log or file parsing triggered by web services and user-facing applications. Here, segregating execution into its own unprivileged account restricts authority minimally necessary.

Compare invoking sed as the web server’s privileged user account, vs. a purpose-made lesser-privilege ID. The latter has much weaker system access if compromised via injection flaws. Integrating this with AppArmor policies thus graduates security.

Chroot Jails for Additional Protection

For high-risk use cases like parsing untrusted content, an additional layer of protection can enforce the sed process inside a chroot jail – an isolated Filesystem subtree. This prevents access to OS resources outside the chroot even if hijacked.

Combining the chroot, a dedicated user ID and AppArmor policies thus locks down sed tightly. The chroot scope can be minimized to exclude unnecessary tools and libraries to reduce attack surface exposed.

Example sed Scripts Demonstrating Safe Practices

Applying the principles outlined, here are some examples of sed scripts handling external input securely:

Sanitizing User Input

#!/bin/sed -f

# Remove whitespace and non-alphabet characters 
s/[^a-zA-Z]//g

# Escape any remaining slashes
s/\//\\/&/g

# Insert sanitized string into search/replace fields
s/foobar/$userInput/

Quoting Interpolated Variables

#!/bin/sed -f

# Quote string literals to avoid re-interpretation 
s/foo/'$userString'/

# Use different delimiters to avoid self-closing substitution
s,foobar,$userInput,  

Disallowing External Program Execution

#!/bin/sed -f

# Omit commands allowing external execution like r, w, !
# Use sed's native feature set only 

s/old/$userInput/new

AppArmor Profile for sed

#include 

/usr/bin/sed {

  # Restrict to approved input/output files
  /var/log/app/*.log r,

  # Allow writes only to output directory
  /var/cache/out/*.txt w,

  # Block other Filesystem access
  deny / rw,

  # No network connectivity
  deny network,

  # Restrict executable mappings
  /usr/bin/sed mr,

}